Resolve ontimeout and onerror

Used 'unresolved' function to resolve the faviconPromise with an empty string

lib/Utils.js

@@ -21,7 +21,7 @@ const bmsvc = Cc["@mozilla.org/browser/nav-bookmarks-service;1"].getService(Ci.n
 const ios = Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService);
 
 // timeout for fetching favicon
-var DEFAULT_TIMEOUT = 3000;
+let DEFAULT_TIMEOUT = 3000;
 
 /**
  * Merge obj2 into obj1.
@@ -238,6 +238,9 @@ function propertyComparator(property) {
 function _getFavicon(faviconURL) {
   let faviconPromise = Promise.defer();
   let xhr = new XMLHttpRequest();
+  let unresolved = function() {
+    faviconPromise.resolve("");
+  }
   xhr.mozBackgroundRequest = true;
   xhr.onload = function() {
     if (this.status == 401 || this.status == 404) {
@@ -248,6 +251,8 @@ function _getFavicon(faviconURL) {
   };
   xhr.open("get", faviconURL, true);
   xhr.timeout = DEFAULT_TIMEOUT;
+  xhr.ontimeout = unresolved;
+  xhr.onerror = unresolved;
   xhr.send();
   return faviconPromise.promise;
 }